The present invention relates to electronic circuitry for generating special effects in color television and, more particularly, to systems for replacing selected portions of a foreground image with a background image when desired.
Various systems exist for keying out selected portions of a foreground image such as may be provided by a foreground television camera and replacing the keyed out portions with a background image such as may be provided by a background television camera. Examples of such systems are provided by U.S. Pat. No. 3,778,542 for Blue Screen Travelling Matte System granted to L.C. Hanseman on Dec. 11, 1973 and U.S. Pat. No. 3,560,638 granted to Ole Skrydstrup and John D. Ross for Chroma Keying System Utilizing Remote Controlled Chroma Keyer on Feb. 2, 1971.
The Hanseman patent provides an example of a travelling matte system in which the foreground and background signals are algebraically combined such as by use of suppression networks to produce the composite signal. In the particular arrangement disclosed in the Hanseman patent a color difference signal is derived by subtracting the sum of the red and green components of the foreground signal from twice the value of the blue component. The foreground signal is selectively suppressed by a network which acts to subtract the blue component from the total signal. The background signal components are applied to multipliers where they are selectively suppressed by the color difference signal.
The Skrydstrup patent discloses a keying system in which the output is effectively switched between the foreground and background to generate the composite signal. In one embodiment of this invention the nonencoded camera outputs are matrixed to form, R, B, and Y (luminance) signals. These signals are passed through multipliers whose control ports have a d.c. input proportional to the sine and cosine of a hue applied thereto. The outputs of the multipliers are summed to produce the key signal V.sub.k =(R-Y) sin .theta.+(B-Y) cos .theta.. The key signal is subsequently delayed by the amount necessary to bring it into the correct time relationship with the encoded background and foreground camera signals. The key signal is then amplified and clipped before it is used to switch between the background and foreground cameras. A clip control is provided to set a threshold below which a key signal is not produced.
The optimum setting of the clip control is dependent upon the ambient lighting conditions in the television production studio. For example, in a studio using a blue background where illumination is uneven, the clip control may have to be set near zero so that all the blue portions of the scene produce a key signal. Thus, it would be possible for the chroma keyer to produce a key from undesired monochrome portions of the foreground signal. These monochrome portions include very dark shadows and bright specularities caused by reflections.